In the process of drilling a well to a fluid-bearing subterranean formation, a first, relatively large borehole is drilled to a depth of about 100 to 200 meters. A tubular well casing, commonly referred to as surface casing is then lowered into the initial, large diameter wellbore and is cemented in place by a process in which a cement slurry is forced downwardly through the central bore of the casing to the bottom of the casing and then upwardly within the annulus created by a outer surface of the casing and the wellbore wall, the cement being forced through the annulus to the surface. Upon setting of the cement, the casing is rigidly located within the borehole and drilling proceeds using a smaller diameter drill bit to pass through the casing to the bottom of the original borehole and downwardly to the desired formation. In a typical well drilling operation, several strings of casing, each of a reduced diameter from that of the preceding casing string is run into the open hole and cemented in place.
It is common in some drilling environments to pass through formations containing water or hydrocarbon fluids such as gas on the way to the desired depth of the well. Migration of these fluids in the wellbore annulus can cause contamination of one fluid source with another. Of particular concern is the contamination of ground water and the migration of gas which presents a significant environmental and safety hazard.
Hydrostatic forces of a fluid column, such as drilling mud, are usually sufficient to overcome the formation pressure to inhibit any gas migration so long as a fluid capable of transmitting hydrostatic forces is in place in the wellbore. Difficulty arises, however, during the process of the setting or hardening of cement in the cementing of well casing. As a cement sets, it no longer produces or transmits any hydrostatic force which counters the formation pressure tending to release gas into the wellbore. As a result, gas begins to channel through the setting cement and upwardly in the wellbore annulus with the potential of contaminating higher level formations and/or creating an environmental and safety hazard.
With gas formations encountered below the surface casing such as in the second or later casing string, the already set cement locating the surface casing in place acts as an effective plug against such vertical gas migration to the surface. Additionally, the solid surface casing cement can act as a plug against which pressurized fluid can be pumped into the problem areas to squeeze off further gas channelling.
Difficulty arises, however, when a gas-bearing formation is encountered in the first 100 to 200 meters of the wellbore, that is, prior to the setting of the surface casing. Such shallow gas formations have been encountered in the southeastern Alberta and Lloydminster areas of Canada wherein gas-bearing formations as shallow as 50 meters or less have been encountered. Gas channelling in the cementing of surface casing has proven to be a very difficult problem in these areas having shallow gas-bearing formations. Additionally, new laws in these areas now require a cementing process which eliminates such gas migration if it is encountered.
Attempts to use conventional casing cementing techniques do not overcome the gas channelling problem. The main cause of gas channelling is hydrostatic pressure reduction in the setting slurry resulting from fluid loss to the surrounding formation, loss of water through hydration with the cement and chemical contraction of the cement matrix. In this process, the internal pressure within the cement slurry falls to the hydrostatic pressure of water which is typically lower than the formation pressure making gas entry into the wellbore annulus and into and through cement inevitable.
One attempt to overcome this problem has been to use a right-angle setting cement which has an extremely short transition time from a true liquid state to a true solid state. It was theorized that, since the critical time for gas channelling is during the setting transition period for the cement, a right-angle setting cement having a short transition time would overcome the problem. However, right-angle setting cements generally have low long-term compressive strength and begin to break down after drill-out of the surface casing has occurred resulting in gas leakage to the surface.
Another proposed method to overcome the problem of gas leakage from shallow formations is the use of so-called expansive cement in which a gas-generating agent such as aluminum powder is incorporated into the cement slurry to expand it during the transition period. However, insufficient hydrostatic pressure is developed to hold the generated gas in place resulting in the gas simply expanding and bubbling to the surface through the slurry, adding to rather than solving the gas channelling problem.
Another technique involves the step of hesitating the displacement of a conventional cement slurry for a two to three hour period. The increased viscosity of the slurry as it setting caused increased friction pressures which were effective against gas channelling so long as the pressure was transmitted to the gas-bearing zone. However, there was no way to control where the pressure was applied.